The present invention is based on a hydraulic automotive vehicle brake system with a return pump and a precharging pump. Concepts of this type are frequently used where the objective is to retrofit a device for automatic braking to brake systems operating according to the return principle for anti-lock control. Automatic braking, on the one hand, refers to traction slip control operations where brake management is only performed by the wheel brakes of the driven wheels or, on the other hand, to a control of the yaw torque of the vehicle by brake management on individual vehicle wheels. In addition, automatic braking refers to a retardation of a vehicle for defined reasons, for example, in the event of a sensor triggering an alarm signal which indicates a too small distance from a vehicle ahead, and like events.
In brake systems operating according to the return principle for anti-lock control, the return pump used is frequently of the self-priming type in order to permit automatic aspiration of pressure fluid from the pressure fluid supply reservoir for automatic braking operations. At low temperatures, however, the viscosity of the pressure fluid used, i.e., brake fluid, is high so that it is very difficult for the return pump to provide the necessary suction capacity.
Sharp throttling effects are caused by the master cylinder when fluid is aspirated from the supply reservoir by way of the brake line and the master cylinder. In addition, a pressure gradient is involved with long pipe lines and bores.
Therefore, it has frequently been disclosed to use a precharging pump. Partly, the return pumps in brake systems of the above type are not of the self-priming type. Partly, however, the precharging pump has only a supporting effect, and the return pump remains the self-priming type.
There are different proposals how to connect the pressure side of the precharging pump to the suction side of the return pump. It must be ensured in any case that the pilot pressure generated by the precharging pump will propagate to the suction side of the return pump and is prevented from discharging into an unpressurized reservoir. Also, the reduction of residual pressure must be ensured when the pumps are disconnected. A frequent problem encountered is that the precharging pump continues running even after its disconnection, with the result that additional valves are required which are opened to discharge the developing residual pressure.
An object of the present invention is to provide a brake system of the type mentioned hereinabove having a precharging concept which is simple and inexpensive, but operates effectively.